In an image forming apparatus, a photosensitive belt unit is important in terms of print quality and running cost. When a pressing against and retraction from a photosensitive belt by a developing device is repeatedly carried out at a specific position on the photosensitive belt, an abrasion, caused by a pressing and retraction of a development roller of the developing device, occurs at a specific position on a surface of the photosensitive belt. When printing of A4 size (a paper length of 297 mm) is continuously carried out, the developing device repeatedly contacts and retracts from the photosensitive belt before or after an A4 size image region, thereby scratching a surface of the photosensitive belt before or after the A4 size image region. Thereafter, when printing of legal size (a paper length of 356 mm) which has a longer image region than the A4 size one is carried out, the scratch made on the photosensitive belt enters the image region, and the trace of the scratch on the photosensitive belt appears in a printed image.
In the related art, in order to prevent the image defect, the range in which the developing device is brought into contact with the photosensitive belt is made wider than a maximum image region of the specifications of a multicolor image forming apparatus, thereby preventing an effect of the abrasion on the photosensitive belt from appearing in the image.
FIG. 4 is a view showing a region in which a developing device in the existing multicolor image forming apparatus is pressed against a photosensitive belt. A developing device pressing region 26, in which a contact starts at a contact start position 28 and ends at a contact end position 27, is set to one and the same fixed place which is wider than a maximum image region and narrower than a portion between joints 19 and 19. The reason is to prevent the occurrence of an image defect resulting from abrasions 23 which occur at the contact start position 28 and the contact end position 27 on the photosensitive belt 2.
FIG. 5 is a sectional view of an aged condition of the contact start position and the contact end position on the photosensitive belt 2 in the existing method. The photosensitive belt 2 includes a substrate 30, an evaporated aluminum layer 31 and an organic photoconductive layer 32. When the abrasions 23 on the photosensitive belt 2 become deep, they pass through the organic photoconductive layer 32 and reach the evaporated aluminum layer 31. This causes dielectric breakdown in the portion of the abrasions 23, thus disabling a printing operation.